Showing papers in "Journal of Ecology in 2003"

Carbon limitation in trees

Abstract: 1 The ongoing enrichment of the atmosphere with CO2 raises the question of whether growth of forest trees, which represent close to 90 global biomass carbon, is still carbon limited at current concentrations of close to 370 p.p.m. As photosynthesis of C3 plants is not CO2 -saturated at such concentrations, enhanced `source activity` of leaves could stimulate `sink activity` (i.e. growth) of plants, provided other resources and developmental controls permit. I explore current levels of non-structural carbon in trees in natural forests in order to estimate the potential for a carbon-driven stimulation of growth. 2 The concentration of non-structural carbohydrates (NSC) in tree tissues is considered a measure of carbon shortage or surplus for growth. A periodic reduction of NSC pools indicates either that carbon demand exceeds con-current supply, or that both source and sink activity are low. A steady, very high NSC concentration is likely to indicate that photosynthesis fully meets, or even exeeds, that needed for growth (surplus assimilates accumulate). 3 The analysis presented here considers data for mature trees in four climatic zones: the high elevation treeline (in Mexico, the Alps and Northern Sweden), a temperate lowland forest of central Europe, Mediterranean sclerophyllous woodland and a semideciduous tropical forest in Panama. 4 In all four climatic regions, periods of reduced or zero growth show maximum C-loading of trees (source activity exceeding demand), except for dry midsummer in the Mediterranean. NSC pools are generally high throughout the year, and are not significantly affected by mass fruiting episodes. 5 It is concluded that, irrespective of the reason for its periodic cessation, growth does not seem to be limited by carbon supply. Instead, in all the cases examined, sink activity and its direct control by the environment or developmental constraints, restricts biomass production of trees under current ambient CO2 concentrations. 6 The current carbohydrate charging of mature wild trees from the tropics to the cold limit of tree growth suggests that little (if any) leeway exists for further CO2-fertilization effects on growth.

Indices of plant competition

Abstract: Summary 1 Mathematical indices, formed by combining several primary measures, can help researchers summarize, interpret and display results from plant competition experiments. This essay compiles and discusses more than 50 indices that have been used in studies of plant competition. 2 Indices allow researchers to quantify and express several attributes of plant competition, including competition intensity and importance, competitive effects and responses, and the outcome of competition. 3 Several qualities should be considered when selecting a competition index for use. These include specificity and clarity of meaning, mathematical and statistical properties, density dependence, versatility and freedom from size bias. 4 Many indices are based on comparisons of plant performance in mixtures compared with pure stands or control plots. Additional measures of competition are also available from yield-density functions, size distributions and neighbourhood analysis. 5 Indices reflect the consequences of competition. In order to appreciate competitive processes, indices have to be used in conjunction with studies of the chronology, circumstances and composition of plant associations.

Mass tree mortality leads to mangrove peat collapse at Bay Islands, Honduras after Hurricane Mitch

Abstract: Summary 1 We measured sediment elevation and accretion dynamics in mangrove forests on the islands of Guanaja and Roatan, Honduras, impacted by Hurricane Mitch in 1998 to determine if collapse of underlying peat was occurring as a result of mass tree mortality. Little is known about the balance between production and decomposition of soil organic matter in the maintenance of sediment elevation of mangrove forests with biogenic soils. 2 Sediment elevation change measured with the rod surface elevation table from 18 months to 33 months after the storm differed significantly among low, medium and high wind impact sites. Mangrove forests suffering minimal to partial mortality gained elevation at a rate (5 mm year−1) greater than vertical accretion (2 mm year−1) measured from artificial soil marker horizons, suggesting that root production contributed to sediment elevation. Basin forests that suffered mass tree mortality experienced peat collapse of about 11 mm year−1 as a result of decomposition of dead root material and sediment compaction. Low soil shear strength and lack of root growth accompanied elevation decreases. 3 Model simulations using the Relative Elevation Model indicate that peat collapse in the high impact basin mangrove forest would be 37 mm year−1 for the 2 years immediately after the storm, as root material decomposed. In the absence of renewed root growth, the model predicts that peat collapse will continue for at least 8 more years at a rate (7 mm year−1) similar to that measured (11 mm year−1). 4 Mass tree mortality caused rapid elevation loss. Few trees survived and recovery of the high impact forest will thus depend primarily on seedling recruitment. Because seedling establishment is controlled in large part by sediment elevation in relation to tide height, continued peat collapse could further impair recovery rates.

Spatial and temporal variation of biomass in a tropical forest: results from a large census plot in Panama

Abstract: 1 We estimated the dry, living, above-ground biomass (AGB) standing stock and its turnover in a 50-hectare forest plot located in moist tropical forest on Barro Colorado Island, Panama. The estimates were obtained using inventory data collected every 5 years from 1985 to 2000, including measurements of all trees ≥ 1 cm diameter. 2 Four different allometric regressions relating trunk diameter and height with AGB were compared. Based on the most consistent method, we estimated that the Barro Colorado forest holds 281 ±20 Mg ha -1 (1 Mg = 10 3 kg) of AGB, lianas included. A third of the AGB is stored in trees larger than 70 cm in diameter. 3 Stand-level AGB increment (growth plus recruitment) was highest in the period 1985-90 (7.05 ± 0.32 Mg ha -1 year -1 , mean ± 95% confidence limits based on samples of multiple hectares) and smallest in the period 1990-95 (5.25 ± 0. 26 Mg ha -1 year -1 ), while AGB losses were similar during the three intervals (mean 5. 43 ± 0.72 Mg ha -1 year -1 ). This resulted in significant differences in AGB change (defined as increment minus loss) among census intervals; including branchfalls, the AGB of Barro Colorado Island increased in 1985-90 (+0.82 ± 0.84 Mg ha -1 year -1 ), decreased in 1990-95 (-0. 69 ± 0. 82 Mg ha -1 year -1 ), and increased again in 1995-2000 (+0.45 ± 0.70 Mg ha -1 year -1 ). The 15-year average was +0.20 Mg ha -1 year -1 , but with a confidence interval that spanned zero (-0.68 to 0.63 Mg ha -1 year -1 ). 4 Branchfalls and partial breakage of stems had a significant influence on the AGB changes. They contributed an average of 0.46 Mg ha -1 year -1 to the AGB loss. About 5% of AGB increment was due to trees less than 10 cm in diameter. 5 To test whether the AGB of tropical forests is increasing due to climate change, we propose that in each forest type, at least 10 hectares of forest be inventoried, and that measurements of the small classes (< 10 cm diameter) as well as large size classes be included. Biomass loss due to crown damage should also be estimated.

Ecological aspects of seed desiccation sensitivity

Abstract: Summary 1 The ability of seeds to survive desiccation is an important functional trait and is an integral part of plant regeneration ecology. Despite this, the topic has received relatively little attention from ecologists. In this study, we examine the relationships between seed desiccation tolerance and two important aspects of plant regeneration ecology: habitat and dormancy. This is done by comparative analysis of a data set of 886 tree and shrub species from 93 families. 2 The proportion of species displaying desiccation sensitive seeds declines as the habitat becomes drier, and possibly also cooler, although the latter observation requires cautious interpretation. Desiccation sensitivity is most common in moist, relatively aseasonal vegetation zones, but is infrequent in, though not absent from arid and highly seasonal habitats. 3 The highest frequency of desiccation sensitivity occurs in non-pioneer evergreen rain forest trees, although 48% of the species examined have desiccation tolerant seeds. In contrast, all pioneer taxa within the data set have drying tolerant seeds. 4 Desiccation sensitivity is more frequent in seeds that are non-dormant on shedding ( c. 31%), than dormant ( c. 9%). Highest frequencies of drying tolerance occur in seeds with physical or combinational dormancy, at 99% and 100%, respectively. 5 Although there is an association between non-dormancy and desiccation sensitivity in both tropical and temperate zones, the relationship does not appear to be causal. 6 Wo rking from the hypothesis that seed desiccation sensitivity represents a derived state in extant species, we use the results to investigate and discuss possible ecological trade-offs and associated fitness advantages. These may explain the hypothesized repeated loss of this trait. The frequent association between large seed size and desiccation sensitivity is also considered.

Response of forest plant species to land-use change: a life-history trait-based approach

Abstract: Summary 1 Classifying species by shared functional characteristics is important if common functional response groups are to be identified among different taxa. 2 We investigated plant traits that determine the response of forest plant species to land use changes using literature data. Sources from eight European countries and four North-eastern American states, comprising 20 field studies yielded information on 216 forest plant species. For these species, data on 13 life history traits were collected. 3 Trait correlation structure was similar in the European and American data-sets and corresponded well to those described in the literature. The European and American herbs and the European graminoids were clustered into distinct emergent groups based on their plant traits. The profiles of the European and American emergent groups were similar. 4 Herb species belonging to emergent groups characterized by low dispersability (i.e. large seeds, low fecundity, unassisted dispersal) were relatively slow colonizers. Dispersability (and not recruitment) seems to be a key factor limiting the colonization of some forest plant species. The relationship between dispersability and colonizing capacity was less clear for graminoids. 5 A life history trait-based approach offers good opportunities to gain insight into the mechanisms behind species response to land-use change.

Habitat association among Amazonian tree species: a landscape‐scale approach

Abstract: 1: Unravelling which factors affect where tropical trees grow is an important goal for ecologists and conservationists. At the landscape scale, debate is mostly focused on the degree to which the distributions of tree species are determined by soil conditions or by neutral, distance-dependent processes. Problems with spatial autocorrelation, sparse soil sampling, inclusion of species-poor sites with extreme edaphic conditions, and the difficulty of obtaining sufficient sample sizes have all complicated assessments for high diversity tropical forests. 2: We evaluated the extent and pervasiveness of habitat association of trees within a 10 000 km 2 species-rich lowland landscape of uniform climate in south-west Amazonia. Forests growing on two non-flooded landscape units were inventoried using 88 floristic plots and detailed soil analyses, sampling up to 849 tree species. We applied singlespecies and community-level analytical techniques (frequency-distributions of presence records, association analysis, indicator species analysis, ordination, Mantel correlations, and multiple regression of distance matrices) to quantify soil/floristic relationships while controlling for spatial autocorrelation. 3: Obligate habitat-restriction is very rare: among 230 tree species recorded in ≥ 10 localities only five (2.2%) were always restricted to one landscape unit or the other. 4: However, many species show a significant tendency to habitat association. For example, using Monte Carlo randomization tests, of the 34 most dominant species across the landscape the distributions of 26 (76.5%) are significantly related to habitat. We applied density-independent and frequency-independent estimates of habitat association and found that rarer species tend to score higher, suggesting that our full community estimates of habitat association are still underestimated due to the inadequate sampling of rarer species. 5: Community-level floristic variation across the whole landscape is related to the variation in 14 of 16 measured soil variables, and to the geographical distances between samples. 6: Multiple regression of distance matrices shows that 10% of the floristic variation can be attributed to spatial autocorrelation, but even after accounting for this at least 40% is attributable to measured environmental variation. 7: Our results suggest that substrate-mediated local processes play a much more important role than distance-dependent processes in structuring forest composition in Amazonian landscapes.

Effects of genetic impoverishment on plant community diversity

Abstract: Summary 1 Established individuals removed at random from populations of 11 long-lived herbaceous species coexisting in a local area of ancient limestone pasture at Cressbrookdale in North Derbyshire were subjected to clonal propagation to produce stocks of genetically identical individuals sufficient to create 36 model communities identical in species composition but widely contrasted in genetic diversity. 2 Three levels of genetic diversity were imposed. In one treatment, all individuals of each species were genetically unique. The second contained four randomly selected genotypes of each species. In the third, there was no genetic diversity in any of the species but each community contained a unique combination of genotypes. 3 Over a period of 5 years the communities were allowed to develop in microcosms containing natural rendzina soil and exposed to a standardized regime of simulated grazing and trampling. The treatments were maintained by the removal of flowers, immature seed-heads and seedlings originating from the seed-bank and seed rain. Point quadrat surveys were used to monitor changes in species composition and diversity in the three experimental treatments. 4 During the experiment a distinction rapidly developed between five canopy dominants and five subordinates, a process that caused the vegetation structure to closely resemble that occurring at Cressbrookdale. 5 A gradual loss of species diversity occurred in all three treatments but by the end of the fifth growing season species diversity was higher in the most genetically diverse communities. 6 Ordination of the 36 communities at intervals over a 5-year period revealed a gradual convergence in the species composition of the 4-genotype and 16-genotype communities and this effect was more strongly developed in the latter. A comparable process was not observed in the 1-genotype communities, suggesting that interaction between particular genotypes of different species in local neighbourhoods may be an essential part of the mechanism that determines the predictable composition of a mature pasture community. 7 It is concluded that, under the conditions of this experiment, genetic diversity within component species reduced the rate at which species diversity declined. The relative importance in this effect of factors such as greater disease resistance and moderated competitive interactions remains uncertain.

Extreme flooding events on the Rhine and the survival and distribution of riparian plant species

Abstract: Summary 1 Summer floods whose severity is affected by flooding duration, submergence depth and underwater light availability, have a large impact on the zonation of riparian plant species. 2 We analysed the range and variability of these flooding components in the River Rhine and quantified their effects on the ability of Arrhenatherum elatius, Achillea millefolium, Rumex acetosa and Rumex crispus, to survive periods of submergence under experimental conditions. 3 Survival characteristics were used to model species’ lower distribution boundaries for extreme and average floods and were compared with the current field distribution. Different light conditions were simulated by implementing three scenarios of suspended load. 4 Extreme deep Rhine floods are characterized by very low median light transmission levels (i.e. below 0.5%). The largest survival responses in the experiment were observed at such low levels (0.4–3.5 µmol m−2 s−1). Strong effects of light were found in R. crispus and A. millefolium, but responses were weaker in A. elatius and R. acetosa. Submergence depth also affected survival, but not as strongly as light. 5 For the flood intolerant species (A. millefolium and A. elatius) the average flood was predicted to have little effect on field distributions under normal light conditions. However, their actual field distributions in 2000 corresponded to the predicted lower boundaries in the extreme years. This suggests that extreme years determine the distributions of these species for many years. The suspended load scenarios significantly modified the predicted lower boundaries in both extreme and average years, implying that plant lower distribution limits may be significantly shifted upwards or downwards depending on the suspended load of the river system. 6 The predicted lower boundaries of the intermediately tolerant R. acetosa and the highly tolerant R. crispus for both extreme and average years were below the actual field distribution in 2000. This suggests that their current distribution is only partly influenced by major flood disturbances and that other factors, either proximate or historical, may play a prominent role.

Plant dispersal in a lowland stream in relation to occurrence and three specific life‐history traits of the species in the species pool

Abstract: Summary 1 The diversity and abundance of viable diaspores trapped at the downstream end of a 15-km lowland stream were quantified and related to five potentially predicting variables: species’ occurrence in the species pool, distance to the nearest stand and the life-history traits seed buoyancy, seed production and plant height. 2 From 126 samples, 106 614 individuals of vascular plants developed, 95.8% from vegetative diaspores and 4.2% from seeds. Among these plants, three free-floating, 12 submerged, 22 emergent (aquatic) and 70 riparian (semi-aquatic and terrestrial) species were recorded, respectively, accounting for 24.3%, 71.9%, 1.2% and 2.6% of the total number of viable diaspores trapped. 3 Of the free-floating, submerged and emergent species, 100%, 98.9% and 23.7% of the diaspores were vegetative, respectively, whereas it was 2.9% for riparian species. 4 Diaspores of 79% of the total number of aquatic species and 40% of riparian species observed in the established vegetation were trapped. Minimal dispersal distances ranged from 0 to 6 km. 5 Multiple regression analysis conducted for submerged species, showed that 71% of the variation in the diaspore pool could be predicted by occurrence of species in the vegetation. For emergent species, seed production and occurrence of species explained 54% of the variation, seed production being most important. Mean seed buoyancy of emergent species was higher than that of the other groups. For riparian species, seed production, occurrence and buoyancy explained 48% of the variation in the diaspore pool. Seed production per plant was the most important variable. Linear regression revealed a negative relationship between distance and abundance of the diaspore pool for submerged and riparian species. 6 We conclude that the occurrence of species in the species pool is a significant predictor for the dispersal of free-floating and submerged aquatics that rely on vegetative propagation. Seed production and buoyancy are of additional importance with regard to emergent aquatics. Riparian species with a limited terrestrial dispersal capacity may largely extend their range by hydrochory. That is, if they produce large amounts of (small) seeds and provided that these can reach the water body. Buoyancy and high frequency and abundance in the established vegetation promote this dispersal capacity as well.

Comparative growth analysis of tropical forest and savanna woody plants using phylogenetically independent contrasts

Abstract: Summary 1 The savanna–forest boundary in the tropics is marked by a discontinuity not only in tree density, but also in species composition, with few tree species regularly occurring in both savanna and forest environments. 2 We performed comparative growth analysis for nine congeneric pairs, each containing one savanna species and one forest species, grown in a factorial design involving two light and two nutrient levels. 3 Contrary to predictions, there was no difference in relative growth rates (RGR) between savanna and forest species. However, there were clear differences in allocation patterns and in phenotypic responses to light intensity. Savanna species allocated more biomass to roots and maintained lower leaf area per unit plant mass (LAR), and lower leaf area per unit leaf mass (SLA). 4 Savanna species also exhibited greater phenotypic plasticity in specific leaf area, leaf area ratio and net assimilation rate in response to light intensity. An increase in LAR in response to shading of savanna species offset a decrease in net assimilation rate per unit leaf area (NAR), such that RGR was largely unaffected. 5 For most plant traits measured, more of the interspecific variation could be attributed to differences among genera than to differences between the two functional types, indicating that these seedling traits are highly conserved within the congeneric species pairs. Many of these traits were correlated to seed mass, which is itself highly conserved within genera.

Self/non-self discrimination in roots

Abstract: Summary 1 Competition usually involves the allocation of limiting resources to non-reproductive functions. Natural selection is expected to favour mechanisms that increase competition with non-self neighbours and limit wasteful competition with self. 2 We used Pisum sativum plants that had two roots and ‘double plants’ with two shoots and two roots that could be either longitudinally separated into two genetically identical but physiologically distinct individuals or left intact. 3 Root development was significantly greater in split-root plants whose neighbours belonged to different plants. Furthermore, root development was relatively greater in the presence of roots of a different plant, regardless of its identity. This discrimination had a vectorial component whereby plants developed more and longer lateral roots towards neighbouring roots of different plants than towards other roots of the same plant. 4 The results thus demonstrate a mechanism of avoiding self-competition that is based on physiological co-ordination among different organs of the same plant rather than on allochemical recognition that depends on genetic differences. 5 The ability to discriminate between self and non-self could be expected to increase resource use efficiency and ecological performance in plants. It could also be expected that tight physiological co-ordination will be selected for between organs of the same plant that have greater probability of being engaged in direct competitive interactions with each other.

Community assembly along proglacial chronosequences in the high Arctic: vegetation and soil development in north‐west Svalbard

Abstract: Summary 1 Community assembly is described for two contrasting high Arctic chronosequences representing glacial regression of up to 2000 years on Svalbard. The chronosequences included a nutrient-poor glacier foreland (Midtre Lovenbre) and a series of nutrientenriched islands (Loven Islands) progressively released from below a tidewater glacier. 2 Soil development and community assembly paralleled proglacial sequences elsewhere but time scales were extended and mature vegetation types comprised species-poor prostrate communities. 3 Initial colonization by Cyanobacteria stabilized soil surfaces and raised nutrient status. Cyanobacteria formed the dominant ground cover (up to 34%) for 60 years, after when they declined. 4 Vascular plants established slowly and represented minor components of ground cover for the first 100 years. Earliest colonizers were often species with ectomycorrhizal associations , followed by mid-successional species that tended to disappear as ground cover increased. Some species present in the mature vegetation at the oldest sites, established only after 60+ years. 5 Species richness of vascular plants increased for c . 100 years, beyond when only occasional species were added. Bryophytes became increasingly dominant with time. 6 Soil development on the Midtre Lovenbre and Loven Island chronosequences was similar after 100 years. Differences subsequently developed, with organic horizon depth, percentage organic matter and water content on the older Loven islands significantly greater than at equivalent Midtre Lovenbre sites. This was associated with increased bryophyte cover but lower vascular plant species richness. One explanation is a slightly more favourable microclimate, coupled with nutrient input from nesting birds. 7 Communities progressively recruit from a limited pool of effectively dispersed species, each with particular ecological requirements that determine their point of entry into the community. A measure of determinism by default is suggested in the way communities assembled. 8 Under climate warming, in the absence of nutrient enrichment, community development will accelerate but will be constrained by nutrient limitations and a restricted species pool. Where nutrients are less limiting, acceleration towards a moss-dominated community is expected, with a lower species richness of vascular plants.

Forecasting plant migration rates: managing uncertainty for risk assessment

Abstract: Summary 1 Anthropogenic changes in the global climate are shifting the potential ranges of many plant species 2 Changing climates will allow some species the opportunity to expand their range, others may experience a contraction in their potential range, while the current and future ranges of some species may not overlap Our capacity to generalize about the threat these range shifts pose to plant diversity is limited by many sources of uncertainty 3 In this paper we summarize sources of uncertainty for migration forecasts and suggest a research protocol for making forecasts in the context of uncertainty

Seed limitation and the regulation of community structure in oak savanna grassland

Abstract: Summary 1 We present results from a long-term sowing experiment conducted in nutrient-poor savanna grassland in eastern Minnesota. We examine the effects of a one-time seed addition of 23 grassland species on plant community dynamics and structure over eight growing seasons. 2 Our goals were to: (i) test the importance of seed availability in regulating plant colonization dynamics and species richness; (ii) assess both the initial effects of sowing on species diversity and community structure and whether these effects increased, persisted or dissipated over the long-term; and (iii) determine the long-term impacts of sown species on the structure and dynamics of the existing community, including effects on species diversity, the abundance of existing (non-sown) species, extinction rate and abundance hierarchy. 3 Sowing led to the successful establishment of several plant species that had not been present in the plots and to increased abundance of other species that were already present. 4 Sowing led to sustained, significant changes in community structure, including increased species richness, increased community evenness, and decreased absolute and relative abundance of non-sown species. Effects of sowing were large and significant 8 years after sowing, revealing the role of seed limitation in these grassland communities. 5 In total, the results suggest that dispersal limitations, species pools and local biotic processes interact to regulate plant community structure.

Floristic patterns along a 43-km long transect in an Amazonian rain forest

Abstract: Summary 1 The floristic variation in Amazonian lowland forests is poorly understood, especially in the large areas of non-inundated (tierra firme) rain forest. Species composition may be either unpredictable as abundances fluctuate in a random walk, more-or-less uniform, or it may correspond to environmental heterogeneity. 2 We tested the three hypotheses by studying the floristic variation of two phylogenetically distant plant groups along a continuous 43-km long line transect that crossed tierra firme rain forest in northern Peru. 3 The observed floristic patterns were compared to patterns in the spectral reflectance characteristics of the forest as recorded in a Landsat TM satellite image. The topography of the transect was measured in the field, and surface soil samples were collected to document edaphic conditions. The two plant groups, pteridophytes and the Melastomataceae, were assessed in 2-m wide and 500-m long sampling units. 4 Floristic similarity (Jaccard index) between sampling units ranged from 0.01 to 0.71 (mean = 0.27), showing that some units were almost completely dissimilar while others were very alike. 5 Spatially constrained clustering produced very similar subdivisions of the transect when based separately on satellite image data, pteriophytes, and Melastomataceae, and the subdivisions were also related to topography and soil characteristics. Mantel tests showed that floristic similarity patterns of the two plant groups were highly correlated with each other and with similarities in reflectance patterns of the satellite image, and somewhat less correlated with geographical distance. 6 Our results lend no support to the uniformity hypothesis, but they partially support the random walk model, and are consistent with the hypothesis that species segregate edaphically at the landscape scale within the uniform-looking forest.

Overlapping resource use in three Great Basin species: implications for community invasibility and vegetation dynamics

Abstract: Summary 1 In the Great Basin of the western United States of America, the invasive annual grass Bromus tectorum has extensively replaced native shrub and bunchgrass communities, but the native bunchgrass Elymus elymoides has been reported to suppress Bromus . Curlew Valley, a site in Northern Utah, provides a model community to test the effects of particular species on invasion by examining competitive relationships among Elymus , Bromus and the native shrub Artemisia tridentata . 2 The site contains Bromus / Elymus , Elymus / Artemisia and monodominant Elymus stands. Transect data indicate that Elymus suppresses Bromus disproportionately relative to its above-ground cover. Artemisia seedlings recruit in Elymus stands but rarely in the presence of Bromus . This relationship might be explained by competition between the two grasses involving a different resource or occurring in a different season to that between each grass and Artemisia . 3 Time reflectometry data collected in monodominant patches indicated that in spring, soil moisture use by Bromus is rapid, whereas depletion under Elymus and Artemisia is more moderate. Artemisia seedlings may therefore encounter a similar moisture environment in monodominant or mixed perennial stands. However, efficient autumn soil moisture use by Elymus may help suppress Bromus . 4 In competition plots, target Artemisia grown with Bromus were stunted relative to those grown with Elymus , despite equivalent above-ground biomass of the two grasses. Competition for nitrogen in spring and autumn, assessed with 15 N tracer, appears to be secondary to moisture availability in determining competitive outcomes. 5 Elymus physiology and function appear to play an important role in determining the composition of communities in Curlew Valley, by maintaining zones free of Bromus where Artemisia can recruit. Key-words : 15 N tracer, Artemisia tridentata , Bromus tectorum , cheatgrass, Elymus elymoides , Great Basin, N uptake, resource competition, semiarid, Sitanion hystrix ,

Nitrogen and phosphorus resorption efficiency and proficiency in six sub-arctic bog species after 4 years of nitrogen fertilization

Abstract: Summary 1 Plant growth at high-latitude sites is usually strongly nutrient-limited. The increased nutrient availability predicted in response to global warming may affect internal plant nutrient cycling, including nutrient resorption from senescing leaves. 2 The effect of increased N supply (10 g N m−2 year−1) on nitrogen and phosphorus resorption efficiency and proficiency in six sub-arctic bog species, belonging to four different growth-forms, was studied in northern Sweden. 3 We hypothesized that while increased N supply would not affect N or P resorption efficiency, it would lead to lower N resorption proficiency (higher N concentrations in leaf litter) and higher P resorption proficiency (lower P concentrations in leaf litter). We also investigated whether the basis on which resorption was expressed (leaf mass, leaf area or unit leaf) influenced the patterns observed. 4 Contrasting with our hypothesis, a general trend of decreased N resorption efficiency occurred in response to increased N supply, but the expected decrease in N resorption proficiency was seen in all species except Betula nana. 5 P resorption efficiency did not change in four species (B. nana, Empetrum hermaphroditum, Eriophorum vaginatum and Rubus chamaemorus) but it decreased in Andromeda polifolia, and increased in Vaccinium uliginosum. P resorption proficiency showed the expected increase in only two species (B. nana and V. uliginosum). 6 Apart from P resorption efficiency, the different calculation methods generally produced similar responses of resorption efficiency and proficiency to N supply. 7 Increased N supply at high-latitude sites clearly leads to more N being returned to the soil through leaf litter production. However, decomposition of such litter will probably become P-limited. 8 Considerable interspecific differences in nutrient resorption proficiency were found, indicating that long-term changes in vegetation composition need to be considered when evaluating plant-mediated effects on ecosystem nutrient cycling in response to increased nutrient supply.

Tree species differentiation in growth, recruitment and allometry in relation to maximum height in a Bornean mixed dipterocarp forest

Abstract: Summary 1 Maximum attainable height varies greatly between tree species in tropical rain forests and covaries with demographic and allometric traits. We examined these relationships in 27 abundant tree species in a mixed dipterocarp forest. These species were monitored over 3 years in two 1-ha plots in western Borneo. A 95-percentile upper height limit was used to represent maximum height, to avoid sample size differences among populations. 2 Average growth rate in trunk diameter was regressed against trunk diameter using a maximum likelihood model and assuming that growth rates were exponentially distributed around the average. Estimated average growth rate at small trunk diameters (up to 11 cm) was independent of maximum height among the 27 species, while the degree of growth reduction at larger diameters was larger for species with smaller maximum height. 3 The recruitment rate efficiency of saplings was negatively correlated with maximum height, regardless of the measure used to assess species abundance. In particular, sapling recruitment per unit basal area declined greatly with increasing maximum height, consistent with model predictions of the traits required for the stable coexistence of species at different heights within the canopy. 4 Allometric analyses showed that understorey species had shorter heights at the same trunk diameter, and deeper crowns at the same tree height, than canopy species. Therefore, understorey species showed adaptive morphology to deep shade. 5 The regressed size-dependent pattern of average growth rate and an assumption that the population was in a steady state readily explained the observed trunk diameter distributions for 21 species among 27 examined. These species, for which the projected size distribution hardly changed when the natural increase or decrease of the population was set at γ = ±0.005 year−1, had mortality rates of more than four times the value of γ.

Hydrological studies on blanket peat: the significance of the acrotelm‐catotelm model

Abstract: 1 Runoff production in blanket peat catchments of the northern Pennine hills, UK was measured through monitoring and experimentation at the plot, hillslope and catchment scale. Water flow from soil pipes was measured in one of the study catchments and overland flow, throughflow and water table were measured in runoff plots; rainfall simulation and tension-infiltrometry provided information on infiltration characteristics of the peat. 2 Saturation-excess overland flow was found to dominate the flashy flow regime; acrotelm stormflow, subsurface pipeflow and macropore flow were also found to be important components of the ecohydrological system. 3 Surface cover, topography and preferential flowpaths were found to be important factors in controlling infiltration and runoff production. 4 Streamflow generation processes that are consistent with the acrotelm-catotelm model are shown to occur in blanket peat with and without Sphagnum cover, but in one of the catchments studied an estimated 10% of the discharge bypassed this route and discharged via pipes. 5 The spatial and temporal variation in hillslope-scale runoff production was demonstrated in the study catchments. This variability in runoff production will be important for hydroecological understanding in peatlands but is often neglected because of over-simplification of processes provided by the traditional two-dimensional acrotelm-catotelm model.

The interacting effects of genetic variation, habitat quality and population size on performance of Succisa pratensis

Abstract: Summary 1We studied the performance of 17 Dutch populations of the perennial Succisa pratensis, in relation to population size, genetic variation and habitat quality We used a path-analytical model to analyse the possible relationships between these variables and performance 2Plants in smaller populations produced fewer seeds per flower head Their seeds had lower germination rates and higher seedling mortality, and more seeds were dormant or non-viable 3Population size was also correlated with genetic measures Small populations had higher inbreeding coefficients than large populations and observed heterozygosity was positively correlated with population size The mean genetic diversity (expected heterozygosity) was relatively high (Hexp = 042), but not correlated with population size 4Less eutrophic habitats appeared to support larger populations High concentrations of NH4 and NO3 in the soil were significantly negatively correlated with population size 5Path-analysis showed that Succisa pratensis is vulnerable to habitat deterioration (eutrophication) Population size was strongly influenced by habitat quality Reduced performance, however, was better explained by direct genetic effects and by habitat deterioration rather than by effects of population size per se Both habitat quality and genetic effects are thus important for population persistence, even in the short term The results suggest that there will be a continuing decline of the small populations, due to deteriorating habitat conditions, decreased genetic variation and a reduced reproductive capacity

There is high potential for the formation of common mycorrhizal networks between understorey and canopy trees in a mixed evergreen forest

Abstract: Summary 1 The patterns of ectomycorrhizal (ECM) host specificity between understorey and canopy trees were investigated in three mixed evergreen forest stands in northern coastal California. ECM root tips from the dominant canopy (Pseudotsuga menziesii) and understorey (Lithocarpus densiflora) trees were sampled from 18 soil cores (six per stand) and identified using molecular techniques (PCR, RFLP, and DNA sequencing of the rDNA ITS region). 2 We found 56 ECM taxa; 17 on both hosts, 27 solely on Pseudotsuga and 12 on Lithocarpus. There were no significant differences in ECM taxon richness or diversity across stands, although ECM taxon richness was significantly higher on Pseudotsuga than Lithocarpus. ECM taxa similarity across stands was low. 3 Multiple-host ECM taxa had significantly higher abundance than single-host ECM taxa and 13 of the 17 multiple-host ECM taxa were present on both hosts within at least one core. Twelve of the 14 cores had at least one ECM taxon that was present on both hosts, although the specific taxon varied between cores and stands. In addition, shared ECM taxa often had unequal relative abundances on the two hosts. 4 Taken together, our results suggest that there is high potential for common mycorrhizal networks to form between Lithocarpus understories and Pseudotsuga canopies in mixed evergreen forests.

Plant zonation in irregularly flooded salt marshes: relative importance of stress tolerance and biological interactions

Abstract: Summary 1 Most studies of salt marsh plant zonation have been at middle to high latitudes of the northern hemisphere, in euhaline or periodically hypersaline marshes with regular tides. In this study, we examined plant zonation in an irregularly flooded marsh in southern Brazil. Pore water characteristics were compared in four vegetation zones across a marsh elevation gradient. Reciprocal transplants between vegetation zones and removal experiments were performed to examine species interactions in low and mid marshes. 2 There was no distinctive gradient of physical stress across the elevation of irregularly flooded low and mid marshes. Moreover, the three dominant plants, Spartina alterniflora, Spartina densiflora and Scirpus maritimus, were able to grow across the entire elevation gradient, i.e. within zones normally occupied by the other species. The only exception was Spartina alterniflora, which was strongly limited by selective herbivory by the crab Chasmagnathus granulata in the Scirpus maritimus zone. 3 Although intra- and interspecific competition reduced growth of all three species, no competitive hierarchy was found in any vegetation zone. 4 These results suggest that, as in tidal marshes of the north hemisphere, competition is important in structuring salt marsh plant communities. In contrast, however, plant zonation in irregularly flooded marshes cannot be explained by displacement of competitive subordinates to physically stressful habitats. The roles of founder effects and selective herbivory in such marshes therefore merits further investigation.

Changes in light and nitrogen availability under pioneer trees may indirectly facilitate tree invasions of grasslands

Abstract: Summary 1 The first trees establishing in grasslands may indirectly favour their seedlings in competition with neighbouring herbaceous vegetation by increasing soil fertility with nitrogen-rich litter and by reducing light levels under their canopies. It is predicted that increasing soil nitrogen availability will accelerate invasion of trees by stimulating their growth more than that of herbaceous species. Decreasing light availability is predicted to increase tree invasion by limiting the growth of herbaceous vegetation more than that of trees (competitive release). 2 We tested these predictions using Chinese tallow tree ( Sapium sebiferum ), which is an aggressive alien invader of grasslands in the southern USA, and Hackberry ( Celtis laevigata ), which is a native tree common in these grasslands. 3 Nitrogen and light were manipulated in two factorial field experiments in grasslands in Texas, USA. In the first, nitrogen was increased and light was decreased in plots with planted Celtis or Sapium seedlings. In the second experiment, light availability to planted Celtis or Sapium seedlings was increased by holding back prairie vegetation. 4 In the first experiment, growth of Celtis and Sapium seedlings increased with nitrogen fertilization while the above-ground biomass of prairie vegetation did not change. Prairie vegetation biomass decreased and tree seedling growth increased under shading. Sapium ’s growth increased dramatically in the treatment with combined nitrogen and shade. Sapium survival decreased when shade was applied. 5 In the second experiment, Sapium growth increased with increased light. Thus, increased Sapium growth at low light levels in the shade experiment was probably a consequence of decreased competitive interference from prairie vegetation, rather than better absolute performance of Sapium in low light levels. 6 These results provide evidence for facilitation as a mechanism involved in tree invasions of grasslands. Changes in resource levels, perhaps in combination with other factors, may explain rapid conversion of grassland communities to woodlands after the first pioneer trees are established. The marked response of Sapium to the combination of nitrogen and shade suggests that these positive feedbacks may be particularly strong for this alien plant species.

An integrated analysis of the effects of past land use on forest herb colonization at the landscape scale

Abstract: Summary 1 A framework that summarizes the direct and indirect effects of past land use on forest herb recolonization is proposed, and used to analyse the colonization patterns of forest understorey herbaceous species in a 360-ha mixed forest, grassland and arable landscape in the Dijle river valley (central Belgium). 2 Fine-scale distribution maps were constructed for 14 species. The species were mapped in 15 946 forest plots and outside forests (along parcel margins) in 5188 plots. Forest stands varied in age between 1 and more than 224 years. Detailed land-use history data were combined with the species distribution maps to identify species-specific colonization sources and to calculate colonization distances. 3 The six most frequent species were selected for more detailed statistical analysis. 4 Logistic regression models indicated that species frequency in forest parcels was a function of secondary forest age, distance from the nearest colonization source and their interaction. Similar age and distance effects were found within hedgerows. 5 In 199 forest stands, data about soils, canopy structure and the cover of competitive species were collected. The relative importance of habitat quality and spatio-temporal isolation for the colonization of the forest herb species was quantified using structural equation modelling (SEM), within the framework proposed for the effects of past land use. 6 The results of the SEM indicate that, except for the better colonizing species, the measured habitat quality variables are of minor importance in explaining colonization patterns, compared with the combination of secondary forest age and distance from colonization sources. 7 Our results suggest the existence of a two-stage colonization process in which diaspore availability determines the initial pattern, which is affected by environmental sorting at later stages.

Dispersal and gene flow of Populus nigra (Salicaceae) along a dynamic river system

Abstract: 1 We used genetic markers to study gene flow of the riparian pioneer tree species Populus nigra along the Drome river (France). This dioecious species is supposed to have more efficient dispersal mechanisms for pollen (wind) and seeds (wind and water) than other trees. 2 Seedlings belonging to the same reproduction/migration event were sampled in 22 riparian forest fragments along the river and their genetic diversity assessed through six nuclear microsatellites. 3 We found a high level of diversity and significant differentiation among populations. The significant isolation by distance allowed us to reject the infinite island model of migration. 4 Gene flow parameters were higher in the upper, mountainous part than in the alluvial plain downstream. There was no accumulation of diversity downstream, indicating migration rates were symmetrical upstream and downstream. This was confirmed by computing individual migration parameters between adjacent populations. 5 The results are discussed with regard to the dispersal mechanisms of seeds and pollen. The discrepancy between potential gene flow and effective gene flow is interpreted as an effect of fragmentation, due to the alteration of the natural dynamics of the riparian ecosystem rather than to physical barriers.

Stimulated growth of Betula pubescens and Molinia caerulea on ombrotrophic bogs: role of high levels of atmospheric nitrogen deposition

Abstract: Summary 1 In order to test whether the observed invasion of ombrotrophic bogs in the Netherlands by Molinia caerulea and Betula pubescens is the result of long-term high nitrogen (N) loads, we conducted a 3-year fertilization experiment with Sphagnum fallax turfs. Six different N treatments were applied ranging from 0 (control) to 4 g N m−2 year−1. 2 During the experimental period, ammonium concentrations in the peat moisture remained very low due to high uptake rates by Sphagnum. Tissue N concentrations in S. fallax showed a linear response to the experimental N addition. Excess N was accumulated as N-rich free amino acids such as arginine, asparagine and glutamine, especially at N addition rates of 0.25 g m−2 year−1 or higher, indicating N-saturation. 3 Despite the high tissue N : P ratio (above 35), above-ground biomass production by Molinia was still stimulated at N addition rates of 4 g m−2 year−1, and foliar nutrient concentrations were unaffected compared to the control. In contrast to Molinia, Betula was unable to increase its above-ground biomass. Foliar N concentrations in Betula were significantly higher at N addition rates of 4 g m−2 year−1 and excess N was stored in foliar arginine, making up 27% of the total N concentration. Evapotranspiration was increased at higher N addition rates due to stimulated total above-ground biomass production of the vegetation. 4 N addition at the actual Dutch deposition rate of 4 g m−2 year−1 stimulated the growth of Molinia in this experiment, providing evidence that the observed dominance of Molinia on ombrotrophic bogs in the Netherlands is caused by high N deposition levels. Based on the observed changes in biomass production and tissue nutrient concentrations, we assume that a long-term deposition of 0.5 g N m−2 year−1, or higher, leads to undesirable changes in species composition and increased risk of desiccation.

Multiple pathways for woody plant establishment on floodplains at local to regional scales

Abstract: Summary 1 The structure and functioning of riverine ecosystems is dependent upon regional setting and the interplay of hydrologic regime and geomorphologic processes. We used a retrospective analysis to study recruitment along broad, alluvial valley segments (parks) and canyon segments of the unregulated Yampa River and the regulated Green River in the upper Colorado River basin, USA. We precisely aged 811 individuals of Populus deltoides ssp. wislizenii (native) and Tamarix ramosissima (exotic) from 182 wooded patches and determined the elevation and character of the germination surface for each. We used logistic regression to relate recruitment events (presence or absence of cohort) to five flow and two weather parameters. 2 Woody plant establishment occurred via multiple pathways at patch, reach and segment scales. Recruitment occurred through establishment on (1) vertically accreting bars in the unregulated alluvial valley, (2) high alluvial floodplain surfaces during rare large flood events, (3) vertically accreting channel margin deposits in canyon pools and eddies, (4) vertically accreting intermittent/abandoned channels, (5) low elevation gravel bars and debris fans in canyons during multi-year droughts, and (6) bars and channels formed prior to flow regulation on the dammed river during controlled flood events. 3 The Yampa River's peak flow was rarely included in models estimating the likelihood that recruitment would occur in any year. Flow variability and the interannual pattern of flows, rather than individual large floods, control most establishment. 4 Regulation of the Green River flow since 1962 has had different effects on woody vegetation recruitment in canyons and valleys. The current regime mimics drought in a canyon setting, accelerating Tamarix invasion whereas in valleys the ongoing geomorphic adjustment of the channel, combined with reduced flow variability, has nearly eliminated Populus establishment. 5 A single year's flow or a particular pattern of flows over a sequence of years, whether natural or man-made, produces different recruitment opportunities in alluvial and canyon reaches, in diverse landforms within a particular river reach, and for Populus and Tamarix. The design of flows to restore riparian ecosystems must consider these multiple pathways and adjust the seasonal timing, magnitude and interannual frequency of flows to match the desired outcome.

Decline in photosynthetic nitrogen use efficiency with leaf age and nitrogen resorption as determinants of leaf life span

Abstract: Summary 1 Cost-benefit models predict that leaf life span depends on its initial photosynthetic rate and construction cost and on the rate of decline in photosynthesis with age. Leaf gas exchange rates and N contents were measured in nine woody evergreen Mediterranean species with different leaf life spans to determine the effects of leaf ageing on photosynthetic N use efficiency (PNUE). N costs of leaf construction were assumed to be in part dependent on N resorption from senescing leaves. 2 Leaf ageing had significant negative effects on photosynthetic rates per unit leaf area. As N content per unit leaf area did not decline until the end of leaf life, PNUE also decreased with age. PNUE generally declined faster in species with a shorter leaf life span. There were no significant interspecific differences in maximum CO2 assimilation rates per unit leaf area and in N resorption that could be related to differences in leaf life span. 3 As PNUE decreases with leaf age, shedding of the older leaves and retranslocation of N to the current year's leaf biomass would result in an increase in the mean instantaneous efficiency of use of the N retranslocated. However, total CO2 assimilation can be improved by such shedding only when the increase in the efficiency of use of the remobilized N compensates for the remaining N lost in the shed leaves. 4 The photosynthesis of the old leaf cohorts exceeded the increase in photosynthesis that would be obtained from the N retranslocated to the younger leaves, given the observed efficiencies of N resorption. The retention of old leaves thus resulted in a higher whole-canopy CO2 assimilation, despite their low PNUE.